This invention relates to electrically actuable devices for positioning members, and particularly to torque motor devices for controlling elements in high speed mechanisms such as computer peripheral devices.
The so-called torque motor or limited angle torquer comprises a permanently magnetized rotor member and a concentric torodial ring about which helical windings are disposed. The rotor member is a two or four pole member on a central rotatable shaft. When current passes through turns of the coils adjacent to the pole tips of the rotor, the rotor is subjected to net torque in one direction or the opposite due to what is known as the Lorenz force. The Lorenz force is that relative force resulting from the interaction between a magnetic field and a current within a conductor. In the torque motor it is usually preferred to have uniform windings and a constant air gap, and to provide torque generating forces at both ends of the rotor. Two windings may be wound in opposite senses on the different halves of the ring core, so that when energized with the same signal current Lorenz forces act in the same rotational sense with oppositely magnetized ends of the rotor. The level of the torque is a function of the amplitude of the energizing current and is constant, if the windings and gap are uniform, until the pole of the rotor encounters turns carrying current that is in a direction to generate opposite torque.
The operative angle for prior art torque motors therefore has been limited to the 180.degree. arc that can be encompassed by a single winding. Moreover, the effective angle of excursion is reduced by the included angle of the rotor pole. The wider the rotor pole the greater the torque due to the consequent increase in interaction region, but the smaller the angle before the rotor leaves the edge of the controlling winding and torque becomes non-linear.
The advantages of this arrangement for high precision, high speed, control systems are many. Because magnetic fields are employed the torque exerted is continuous within an arc of influence, and there is no cogging or ripple effect because the air gap is constant. The shaft is directly driven, and substantial torque can be exerted to give rapid acceleration to a position that is resolvable very precisely, with no theoretical limits on resolution. Also, friction can be minimal, rotor weight and inertia are low, torque-to-inertia ratio is high and the motor is brushless so that low power levels and DC signals can be employed. Consequently these positioners are used in a wide variety of applications, such as arm positioners in random access memories (particularly disk drives for data processors), antenna positioners, scanners, valve controls and a wide variety of other advanced control mechanisms.
The fundamental design problem, however, in the operation of these prior art torque motors is that the angular excursion within which torque is linear is inversely related to torque, when the rotor angle at the pole tips is widened or narrowed. The maximum included angle for the rotor poles is 90.degree., but this can result in an unacceptably small range of linear operation. Most typically, the angular excursion is limited to about .+-.60.degree. in most commercial motors of this type. Useful torque levels for rapid response are substantially less, being in the range of .+-.20.degree. to .+-.40.degree., depending on the design. While an intermediate gearing or belt mechanism may sometimes be used to increase the excursion of the controlled device these elements add inertia, require space and can also introduce unacceptable tolerance variations. The torque motor and shaft may be made in a larger size, at greater cost, to provide the needed range of displacements, but again inertia, volume or expense can be unacceptable. There are many instances in which a torque motor is needed that has an unlimited choice of excursion angles and a small size for the particular application.